Bone cements are widely used in orthopedic surgery, for example, for the replacement of hip joints, in craniofacial reconstruction procedures, in dentistry and in similar applications. In the past, such cements have commonly taken the form of polyacrylate compositions polymerized in situ, for instance, to anchor bone prostheses in place. Typically, such systems include two interreactive components, one a powder comprised of poly(methyl methacrylate), PMMA, and the other, liquid methyl methacrylate, MMA. The two components are intermixed to form a kneadable dough which is introduced, for example in the case of a hip replacement, into a prepared cavity in a femur, the metal alloy prosthesis being inserted into the cavity thereafter. The "dough" can be manipulated by the physician for a period of about seven minutes, at which time, it will have polymerized to a viscosity such that it becomes impossible to manipulate it further. A bone prosthesis thus installed is not only firmly set in position, but the bone cement used to embed it acts to distribute stresses operative at the prosthesis-cement interface over a wide area, thereby reducing stress concentrations to a manageable level.
Unfortunately, while bone cements of the type described are characterized by a high degree of strength, they also display certain disadvantageous characteristics which the invention is designed to overcome. For instance, one difficulty frequently encountered with poly(methyl methacrylate) bone cements of the prior art resides in the fact that while poly(methyl methacrylate) exhibits high strength, the polymer is relatively brittle. Consequently, prosthesis implants so fastened tend over time to become loosened due to the stresses constantly being experienced. Loosening is a major cause of failures of this type of reconstruction.
In addition, however, the constant wear and tear to which such implants are exposed tends to result in erosion of the brittle polymer, accompanied by the concurrent formation of small wear particles. Eventually, therefore, the anchoring interfaces become weakened and insufficient to function properly, leading to ultimate failure of joints so established. In this regard, currently available data indicates that remedial work necessitated as a consequence of joint replacement failures may be required in as many as about 4.5% of such cases.
In view of the preceding, therefore, it is a first aspect of this invention to provide a superior bone cement composition for use in medical procedures.
It is a second aspect of this invention to provide bone cement compositions that are tougher than the polyacrylate compositions presently in use.
Another aspect of this invention is to provide bone cements that avoid the brittleness and concomitant inferior fatigue characteristics that are associated with poly(methyl methacrylate) compositions of the prior art.
Yet another aspect of this invention is to provide novel three-arm, or star polyisobutylene compositions comprising methacrylate tritelechelic polyisobutylenes.
An additional aspect of this invention is to provide a poly(methyl methacrylate) polymer toughened by having been cross-linked with methacryloyl-capped, three-arm polyisobutylene compositions.
A further aspect of this invention is to provide a new type of semi-simultaneous interpenetrating network useful in preparing toughened bone cements.
The preceding and additional aspects of this invention are provided by a polymeric composition consisting essentially of tris(.omega.-methacryloyl) polyisobutylene.
The preceding and other aspects of the invention are provided by a composition comprising poly(methyl methacrylate) partially cross-linked with a telechelic compound consisting of tris(.omega.-acryloyl) polyisobutylene.
The preceding and further aspects of the invention are provided by a bone cement comprising the reaction product of (1) a polymeric composition according to the preceding paragraph in which the telechelic compound is tris(.omega.-methacryloyl) polyisobutylene, and (2) methyl methacrylate.
The preceding and yet additional aspects of the invention are provided by a process for making compositions of the type described in the penultimate paragraph in which the tris(.omega.-methacryloyl) polyisobutylene and the methyl methacrylate are agitated until a gelled reaction product begins to form, and then reacted further without agitation until a solid polymeric product is formed.
The preceding and still further aspects of the invention are provided by a semi-simultaneous interpenetrating network of a polymeric composition comprising poly(methyl methacrylate) cross-linked with tris(.omega.-methacryloyl) polyisobutylene, the polymeric composition being interpenetrated with a poly(methyl methacrylate) homopolymer.